Fixing bugs with action cancellations

[orduno]

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Basic Info

Info	Please fill out this column
Ticket(s) this addresses	#911
Primary OS tested on	Ubuntu 18.04
Robotic platform tested on	Turtlebot3 gazebo

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Description of contribution in a few bullet points

Made a few improvements to SimpleActionServer, mainly to address #911:

• Check if the goal is active before canceling or aborting.
• Cancel only if the client has requested so, otherwise abort.
• Remove separate cancel and abort methods, instead have one terminate_goals which calls the appropriate method depending on the state of the goal.

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Future work that may be required in bullet points

[mhpanah]

Looks good.

[codecov]

Codecov Report

Merging #918 into master will increase coverage by 8.66%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #918      +/-   ##
=========================================
+ Coverage   21.13%   29.8%   +8.66%     
=========================================
  Files         184     191       +7     
  Lines        9448    9934     +486     
  Branches     2313    3272     +959     
=========================================
+ Hits         1997    2961     +964     
+ Misses       6311    5162    -1149     
- Partials     1140    1811     +671

Impacted Files	Coverage Δ
...v2_util/include/nav2_util/simple_action_server.hpp	18.1% <0%> (-16.52%)	⬇️
...rc/navigation2/nav2_motion_primitives/src/main.cpp	16.66% <0%> (-8.34%)	⬇️
...rc/navigation2/nav2_motion_primitives/src/spin.cpp	11.66% <0%> (-3.23%)	⬇️
...navigation2/nav2_motion_primitives/src/back_up.cpp	12.82% <0%> (-2.81%)	⬇️
src/navigation2/nav2_util/test/test_actions.cpp	50% <0%> (-0.5%)	⬇️
src/navigation2/nav2_util/src/lifecycle_node.cpp	62.5% <0%> (ø)	⬆️
...navigation2/nav2_bt_navigator/src/bt_navigator.cpp	0% <0%> (ø)	⬆️
...b_controller/dwb_controller/src/dwb_controller.cpp	0% <0%> (ø)	⬆️
...c/navigation2/nav2_world_model/src/world_model.cpp	0% <0%> (ø)	⬆️
...tion2/nav2_costmap_2d/src/costmap_2d_publisher.cpp	0% <0%> (ø)	⬆️
... and 47 more

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